Paper coating compositions comprising a polymer dispersion from room temperature liquid and gaseous monomers

ABSTRACT

Paper coating compositions comprise inorganic pigments and an aqueous polymeric dispersion comprising dispersed polymeric particles. The polymers are obtainable by polymerization of a first monomer that is liquid at room temperature, has a boiling point of at least 50° C. and a glass transition temperature of at least 20° C. as homopolymer, and a second monomer that is gaseous at room temperature, has a boiling point of below 0° C. and a glass transition temperature of below −30° C. as homopolymer. The dispersed polymeric particles have an average size of below 150 nm and a glass transition temperature in the range from −10 to +30° C.

The invention concerns paper coating compositions comprising inorganic pigments and an aqueous polymeric dispersion comprising dispersed polymeric particles. The polymers are obtainable by polymerization of certain room temperature liquid monomers and certain room temperature gaseous monomers. The dispersed polymeric particles have an average size of below 150 nm and a glass transition temperature in the range from −10 to +30° C.

Paper coating compositions besides water generally comprise pigments, binders and auxiliaries to establish the requisite rheological properties, for example thickeners. Paper coating compositions endow raw papers with the desired mechanical and visual properties. The binder is intended to fix the pigments to the paper and ensure coherency in the coating obtained. Binders with very high binding power are therefore desirable. And there are some polymeric binders which do have high binding power, but they tend to yellow, which is undesirable in many applications on paper. There are other polymeric binders that do have high binding power and/or high yellowing resistance, but they have an unpleasant odor due to the components used to make them. Binders hitherto used in paper coating agents are not yet ideal in all desirable properties.

It is an object of the present invention to provide paper coating compositions combining very high binding power with very high yellowing resistance.

The invention provides a paper coating composition comprising

-   -   (i) inorganic pigments and     -   (ii) an aqueous polymeric dispersion comprising dispersed         polymeric particles, wherein the polymer is obtainable by         polymerization of         -   (a) at least one first monomer that is liquid at room             temperature (20° C.), has a boiling point of at least 50° C.             and a glass transition temperature of at least 20° C. as             homopolymer, and         -   (b) at least one second monomer that is gaseous at room             temperature (20° C.), has a boiling point of below 0° C. and             a glass transition temperature of below −30° C. as             homopolymer,             wherein the dispersed polymeric particles have an average             size of below 150 nm and a glass transition temperature in             the range from −10 to +30° C.

The invention also provides a process for coating paper or board, which process comprises

-   -   providing a paper coating composition according to the         invention; and     -   applying the paper coating composition to at least one surface         of paper or board.

The invention also provides paper or board coated with a paper coating composition according to the invention.

The dispersed polymeric particles have an average size which according to the invention is below 150 nm and preferably in the range from 80 to below 150 nm. The average diameter of the polymeric particles can be measured by hydrodynamic chromatography (HDC). In HDC, a colloidal sample elutes from a size-exclusion separation column sorted according to hydrodynamic radius. The eluent comprises salt, nonionic surfactants and anionic surfactants. Elution time is calibrated with PS calibration latices. The measurement range extends from 15 nm to 1200 nm—larger components are filtered out and not detected. Diameter and weight fractions can be measured to an accuracy of 3%. The fractions are weighted using the UV absorption at 254 nm.

100 parts by weight of total monomers used in the polymerization comprise for example 4.8 to 95 parts by weight, preferably 9.8 to 75 parts by weight and especially 14.8 to 60 parts by weight of at least one monomer of group (a) and 4.8 to 95 parts by weight, preferably 24.8 to 90 parts by weight and especially 39.8 to 85 parts by weight of at least one monomer of group (b).

The first monomer (a) is preferably selected from vinylaromatic compounds and vinyl esters. Examples of vinylaromatic compounds are styrene, a-methylstyrene and vinyltoluene. Styrene, methylstyrene and mixtures thereof are preferable. It is particularly preferable to use styrene. Vinyl acetate is an example of a suitable vinyl ester.

The second monomer (b) is preferably selected from mono- or polyethylenically unsaturated C2 to C4 hydrocarbons, for example 1,3-butadiene, isoprene and ethylene. Of this group of monomers, it is preferable to use 1,3-butadiene or ethylene.

In one preferable embodiment of the invention, the at least one first monomer (a) is selected from styrene and vinyl acetate and the at least one second monomer (b) is selected from 1,3-butadiene and ethylene. Preferably, the polymer is selected from the group consisting of polymers formed from ethylene and vinyl acetate to an extent of at least 60% by weight, preferably at least 80% by weight, polymers formed from styrene and butadiene to an extent of at least 60% by weight, preferably at least 80% by weight, and mixtures thereof.

Useful additional monomers for preparing the polymers optionally include ethylenically unsaturated acids in an amount of preferably 0.1 to 10 parts by weight, preferably 0.2 to 8 parts by weight or 1 to 6 parts by weight, based on 100 parts by weight of monomers. Examples of ethylenically unsaturated acids are ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulfonic acids and vinylphosphonic acid. Ethylenically unsaturated carboxylic acids used are preferably α,β-monoethylenically unsaturated mono- and dicarboxylic acids having 3 to 6 carbon atoms in the molecule. Examples thereof are acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid and vinyllactic acid. Useful ethylenically unsaturated sulfonic acids include for example vinylsulfonic acid, styrenesulfonic acid, acrylamidomethylpropanesulfonic acid, sulfopropyl acrylate and sulfopropyl methacrylate. Ethylenically unsaturated acids can be used in the polymerization as free acids and also after partial or complete neutralization with suitable bases. It is preferable to use aqueous sodium hydroxide solution, aqueous potassium hydroxide solution or ammonia as neutralizing agents.

Yet further ethylenically unsaturated compounds other than the hereinbefore mentioned monomers can be used, optionally. The further monomers can be used in amounts of 0 to 15 parts by weight, for example in an amount of 0.1 to 15 parts by weight or of 0.5 to 10 parts by weight per 100 parts by weight of monomer mixtures. Examples of further monomers are unsaturated nitriles such as, for example, acrylonitrile and methacrylonitrile; ethylenically unsaturated carboxamides such as, for example, acrylamide and methacrylamide; vinyl esters of saturated C1 to C18 carboxylic acids; esters of acrylic acid and of methacrylic acid with monohydric C₁ to C₁₈ alcohols such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, pentyl acrylates, pentyl methacrylates, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, allyl esters of saturated carboxylic acids, vinyl ethers, vinyl ketones, dialkyl esters of ethylenically unsaturated dicarboxylic acids, N-vinylpyrrolidone, N-vinylpyrrolidine, N-vinylformamide, N,N-dialkylaminoalkylacrylamides, N,N-dialkylaminoalkylmethacrylamides, N,N-dialkylaminoalkyl acrylates, N,N-dialkylaminoalkyl methacrylates, vinyl chloride and vinylidene chloride. The further monomers are preferably monoethylenically unsaturated. However, it is also possible to use polyunsaturated monomers, especially crosslinking monomers having two or more ethylenic double bonds, for example alkanediol diacrylates, e.g., butanediol diacrylate.

One embodiment utilizes

-   -   (a) 4.8 to 95 parts by weight, preferably 9.8 to 75 parts by         weight, of styrene or methylstyrene,     -   (b) 4.8 to 95 parts by weight, preferably 24.8 to 90 parts by         weight, of butadiene,     -   (c) 0.1 to 10 parts by weight of at least one ethylenically         unsaturated acid, preferably selected from acrylic acid,         methacrylic acid, itaconic acid, maleic acid, fumaric acid,         crotonic acid, vinylacetic acid, vinyllactic acid, vinylsulfonic         acid, styrenesulfonic acid, acrylamidomethylpropanesulfonic         acid, sulfopropyl acrylate and sulfopropyl methacrylate, and     -   (d) 0 to 15 parts by weight of at least one other         monoethylenically unsaturated monomer,         wherein the parts by weight of monomer each sum to 100.

One embodiment utilizes

-   -   (a) 4.8 to 95 parts by weight, preferably 39.8 to 85 parts by         weight, of vinyl acetate,     -   (b) 4.8 to 95 parts by weight, preferably 14.8 to 60 parts by         weight, of ethylene, and     -   (c) 0 to 10 parts by weight of at least one other         monoethylenically unsaturated monomer,         wherein the parts by weight of monomer each sum to 100.

The polymers are for example obtainable by emulsion polymerization. This may be carried out by initially charging some of the initiator and some, for example from 1% to 10% by weight, of total monomers to be polymerized, in an aqueous medium. The remaining monomers and the rest of the initiator are then metered, separately, into the initial charge, once the polymerization has started, under polymerization conditions. The polymerization can also be carried out in the presence of protective colloids such as, for example, polyvinyl alcohol or a degraded starch, for example a degraded natural starch having an intrinsic viscosity η_(i) of 0.02 to 0.06 dl/g. The term polymerization conditions is to be understood as meaning that the reaction mixture in the initial charge has been heated to the requisite temperature at which the polymerization proceeds. These temperatures range for example from 80 to 130° C. and preferably from 90 to 120° C. The polymerization is preferably carried out under superatmospheric pressure, for example at pressures up to 60 bar, although copolymers of butadiene are typically polymerized at 2 to 10 bar pressure and copolymers of ethylene preferably at pressures of 40 to 60 bar.

The process of the present invention typically utilizes initiators that form free radicals under the reaction conditions. Examples of suitable polymerization initiators are peroxides, hydroperoxides, hydrogen peroxide, sodium persulfate, potassium persulfate, redox catalysts and azo compounds such as 2,2-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2-azobis(2,4-dimethylvaleronitrile) and 2,2-azobis(2-amidinopropane) dihydrochloride. Examples of further suitable initiators are dibenzoyl peroxide, tert-butyl perpivalate, tert-butyl per-2-ethylhexanoate, di-tert-butyl peroxide, diamyl peroxide, dioctanoyl peroxide, didecanoyl peroxide, dilauroyl peroxide, bis(o-toluoyl) peroxide, succinyl peroxide, tert-butyl peracetate, tert-butyl permaleate, tert-butyl perisobutyrate, tert-butyl perpivalate, tert-butyl peroctoate, tert-butyl perbenzoate, tert-butyl hydroperoxide, azobisisobutyronitrile, 2,2″-azobis(2-methylbutyronitrile), 2,2″-azobis(2,4-dimethylvaleronitrile) and 2,2″-azobis(N,N″-dimethyleneisobutyramidine) dihydrochloride. Preference is given to initiators selected from the group of peroxodisulfates, peroxosulfates, azo initiators, organic peroxides, organic hydroperoxides and hydrogen peroxide. Particular preference is given to using water-soluble initiators, e.g., sodium persulfate, potassium persulfate, ammonium persulfate, sodium peroxodisulfate, potassium peroxodisulfate and/or ammonium peroxodisulfate. The polymerization can also be initiated using high-energy rays such as electron beam rays or by irradiation with UV light.

The initiators are used for example in amounts up to 2% by weight, preferably not less than 0.9% by weight, for example in the range from 1.0% to 1.5% by weight, based on the monomers to be polymerized. Preferably, at least 30% by weight of the initiators are initially charged in the aqueous medium optionally together with protective colloid, and the monomers and also the remaining initiators are metered into this initial charge under polymerization conditions.

The polymer dispersions are prepared by initially charging, for example to a heatable reactor equipped with a mixing device, preferably not less than 30% by weight of total initiator quantity needed and optionally an aqueous solution of one of the protective colloids described above. The amount of initiator in the initial charge is preferably not more than 90% by weight and usually not more than 60% by weight of the total amount required to polymerize the monomers.

The protective colloid effects good dispersal of the monomers and a stabilization of the finely divided polymers formed. The protective colloid undergoes at least partial grafting in the emulsion polymerization and becomes firmly incorporated in the polymer being formed.

Monomer dispersal in the aqueous medium can be augmented by using the protective colloids and/or emulsifiers customarily used as dispersants. A detailed description of further suitable protective colloids appears in Houben-Weyl, Methoden der organischen Chemie, Volume XIV/1, Makromolekulare Stoffe, Georg-Thieme-Verlag, Stuttgart, 1961, pages 411 to 420. Suitable emulsifiers include surface-active substances whose number average molecular weight is typically below 2000 g/mol or preferably below 1500 g/mol, while the number average molecular weight of protective colloids is above 2000 g/mol, for example in the range from 2000 to 100 000 g/mol and especially in the range from 5000 to 50 000 g/mol.

Suitable emulsifiers include for example ethoxylated C₈ to C₃₆ fatty alcohols having a degree of ethoxylation in the range from 3 to 50, ethoxylated mono-, di- and tri- C₄ to C₁₂ alkylphenols having a degree of ethoxylation in the range from 3 to 50, alkali metal salts of dialkyl esters of sulfosuccinic acid, alkali metal and ammonium salts of C₈ to C₁₂ alkyl sulfates, alkali metal and ammonium salts of C₁₂ to C₁₈ alkylsulfonic acids and alkali metal and ammonium salts of C₉ to C₁₈ alkylarylsulfonic acids. Examples of cation-active emulsifiers are compounds having at least one amino or ammonium group and at least one C8-C22 alkyl group. When emulsifiers and/or protective colloids are used as auxiliaries to disperse the monomers, the amounts in which they are used are in the range from 0.1% to 5% by weight, based on the monomers, for example.

The initial charge may further comprise a polystyrene seed, i.e., an aqueous dispersion of finely divided polystyrene having a particle diameter of 20 to 40 nm.

To modify the properties of polymers, the emulsion polymerization may optionally be carried out in the presence of at least one chain transfer agent. Examples of chain transfer agents are organic compounds comprising sulfur in bound form such as dodecyl mercaptan, thiodiglycol, ethylthioethanol, di-n-butyl sulfide, di-n-octyl sulfide, diphenyl sulfide, diisopropyl disulfide, 2-mercaptoethanol, 1,3-mercaptopropanol, 3-mercaptopropane-1,2-diol, 1,4-mercaptobutanol, thioglycolic acid, 3-mercaptopropionic acid, mercaptosuccinic acid, thioacetic acid and thiourea. Further chain transfer agents are aldehydes such as formaldehyde, acetaldehyde and propionaldehyde, organic acids such as formic acid, sodium formate or ammonium formate, alcohols such as more particularly isopropanol and also phosphorus compounds such as sodium hypophosphite. When a chain transfer agent is used in the polymerization, the amount in which it is used in each case is for example in the range from 0.01% to 5% and preferably in the range from 0.1% to 1% by weight, based on the monomers used in the polymerization. Chain transfer agents are preferably metered into the initial charge together with the monomers. However, they may also be wholly or partly included in the initial charge. They can also be added in stages offset relative to the monomers.

The emulsion polymerization is carried out in an aqueous medium. The aqueous medium may comprise for example completely ion-free water or else mixtures of water and a water-miscible solvent such as methanol, ethanol or tetrahydrofuran. To polymerize the monomers, the first step is to prepare an aqueous solution of degraded starch. This solution may optionally comprise a protective colloid and/or an emulsifier in dissolved form and also, optionally, a polystyrene seed. The aqueous solution used as initial charge is preferably heated to the temperature at which the polymerization of the monomers is to take place or to a temperature which is for example 5 to 20° C. below the polymerization temperature before at least 30% by weight of the total initiator quantity required is added to the initial charge. As soon as the particular polymerization temperature desired is reached or within a time span of 1 to 15 minutes and preferably 5 to 15 minutes after reaching the polymerization temperature, the metered addition of the monomers is commenced. They can be for example pumped into the reactor continuously within for example 60 minutes to 10 hours and usually within 2 to 4 hours.

A staged addition of monomers is also possible. In one preferable embodiment of the process according to the present invention, from 1% to 10% by weight of total monomers to be polymerized is initially charged. The monomers are in this case preferably initially charged to the reactor together with the abovementioned constituents before the reactor contents are heated to the polymerization temperature, while at least 30% by weight of the initiator quantity is preferably added shortly before reaching the polymerization temperature, as described above, and then the remaining monomers are added as indicated above. After the polymerization has ended, further initiator may optionally be added to the reaction mixture for a secondary polymerization at the same temperature as the main polymerization or else at a lower or higher temperature. To complete the polymerization reaction, additional stirring of the reaction mixture at the polymerization temperature for 1 to 3 hours, for example, following addition of all monomers will suffice in most cases.

The pH can be for example in the range from 1 to 5 during the polymerization. After polymerization, the pH is adjusted to a value between 6 and 7 for example.

In one embodiment, the solids content of the aqueous polymeric dispersion of the present invention is more than 55% by weight, e.g., at least 60% by weight. Such a solids content can be effected for example through appropriate adjustment of the water quantity used in the emulsion polymerization and/or the monomer quantities.

The aqueous polymeric dispersions of the present invention are useful as binders for producing paper coating compositions.

Paper coating compositions besides water generally comprise pigments, binders and optionally auxiliaries to establish the requisite rheological properties, for example thickeners. The pigments are typically in a state of dispersion in water. The paper coating composition comprises pigments in an amount of preferably at least 80% by weight, for example 80% to 95% by weight or 80% to 90% by weight, based on total solids content. White pigments are contemplated in particular. Suitable pigments include for example metal salt pigments such as, for example, calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate and calcium carbonate, of which carbonate pigments, especially calcium carbonate are preferable. Calcium carbonate may be natural ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), lime or chalk. Suitable calcium carbonate pigments are available for example as Covercarb® 60, Hydrocarb® 60 or Hydrocarb® 90 ME. Examples of further suitable pigments are silicas, aluminas, aluminum hydrate, silicates, titanium dioxide, zinc oxide, kaolin, argillaceous earth, talc or silicon dioxide. Further suitable pigments are available, for example as Capim® MP 50 (Clay), Hydragloss® 90 (Clay) or Talcum C10.

The paper coating composition comprises at least one of the binders described above. The polymer dispersion obtained according to the present invention can be used in the paper coating composition as sole binder or combined with further binders. The most important functions of binders in paper coating compositions are to bind the pigments to the paper and the pigments to each other and to some extent fill voids between pigment particles. For every 100 parts by weight of pigments, the amount of organic binder used (in terms of binder solids, i.e., without water or other solvents which are liquid at 21° C., 1 bar) is for example in the range from 1 to 50 parts by weight and preferably in the range from 1 to 25 parts by weight or in the range from 5 to 20 parts by weight.

Useful binders further include natural-based binders, especially starch-based binders and also synthetic binders which are other than the polymers obtained according to the present invention and are more particularly emulsion polymers obtainable by emulsion polymerization. Starch-based binders are in this context to be understood as meaning any natural, modified or degraded starch. Natural starches may consist of amylose, amylopectin or mixtures thereof. Modified starches may be oxidized starch, starch esters or starch ethers. Hydrolysis can be used to reduce the molecular weight of starch (to obtain degraded starch). Useful degradation products include oligosaccharides or dextrins. Preferred starches are cereal starch, maize starch and potato starch. Particular preference is given to cereal starch and maize starch and very particular preference to cereal starch.

The further synthetic binders other than the polymers obtained according to the present invention preferably consist of so-called main monomers to an extent of at least 40% by weight, preferably to an extent of at least 60% by weight and more preferably to an extent of at least 80% by weight. Main monomers are selected from C1-C20 alkyl (meth)acrylates, vinyl esters of carboxylic acids comprising up to 20 carbon atoms, vinylaromatics having up to 20 carbon atoms, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds, or mixtures thereof. Examples are alkyl (meth)acrylates with a C1-C10 alkyl radical, such as methyl methacrylate, methyl acrylate, n-butyl acrylate, ethyl acrylate and 2-ethylhexyl acrylate. Mixtures of alkyl (meth)acrylates are also suitable in particular. Examples of vinyl esters of carboxylic acids having 1 to 20 carbon atoms are vinyl laurate and vinyl stearate. Examples of nitriles are acrylonitrile and methacrylonitrile. Vinyl halides are chlorine-, fluorine- or bromine-substituted ethylenically unsaturated compounds, preferably vinyl chloride and vinylidene chloride. Examples of vinyl ethers include vinyl methyl ether and vinyl isobutyl ether. Vinyl ethers of alcohols comprising 1 to 4 carbon atoms are preferred. Preferred main monomers are C1-C10 alkyl (meth)acrylates and mixtures of alkyl (meth)acrylates with vinylaromatics, especially styrene.

In addition to main monomers, the emulsion polymer useful as binder may comprise further monomers, for example monomers having carboxylic acid, sulfonic acid or phosphonic acid groups. Carboxylic acid groups are preferable. Examples include acrylic acid, methacrylic acid, itaconic acid, maleic acid and fumaric acid. The level of ethylenically unsaturated acids in the emulsion polymer is generally below 10% by weight and preferably below 8% by weight and at least 0.1% by weight or at least 1% by weight. Further monomers also include, for example, hydroxyl-containing monomers, especially C1-C10 hydroxyalkyl (meth)acrylates, or amides such as (meth)acrylamide.

When synthetic binders are used, natural binders such as starch can also be used, but they are not absolutely necessary.

Paper coating compositions according to the present invention may additionally comprise further additive and assistant materials, for example fillers, cobinders and thickeners to further optimize viscosity and water retention, optical brighteners, dispersants, surfactants, slip agents (e.g., calcium stearate and waxes), neutralizing agents (e.g., NaOH or ammonium hydroxide) for pH control, defoamers, deaerators, preservatives (e.g., biocides), flow control agents, dyes (especially soluble dyes), etc. Useful thickeners in addition to synthetic polymers (crosslinked polyacrylate for example) include more particularly celluloses, preferably carboxymethylcellulose. Examples of optical brighteners are fluorescent or phosphorescent dyes, especially stilbenes.

The paper coating composition is preferably an aqueous paper coating composition; water is present therein particularly due to the make-up form of the constituents (aqueous polymer dispersions, aqueous pigment slurries); the desired viscosity can be set by adding further water. Customary solids contents for paper coating compositions are in the range from 30% to 70% by weight. The paper coating composition pH is preferably adjusted to values in the range from 6 to 10 and especially in the range from 7 to 9.5.

The invention provides in one embodiment a paper coating composition wherein the polymers of the aqueous polymeric dispersion are used in an amount of 1 to 50 parts by weight, based on the total amount of pigments, and the pigments are comprised in an amount of 80 to 95 parts by weight, based on total solids content.

Preferably, the pigments are selected from the group consisting of calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate, calcium carbonate, silicas, aluminas, aluminum hydrate, silicates, titanium dioxide, zinc oxide, kaolin, argillaceous earth, talc and silicon dioxide.

Preferably, the paper coating composition further comprises at least one assistant selected from the group consisting of thickeners, further polymeric binders, cobinders, optical brighteners, fillers, flow control agents, dispersants, surfactants, slip agents, neutralizing agents, defoamers, deaerators, preservatives and dyes.

The invention also provides paper or board coated with a paper coating composition according to the invention and a process for coating paper or board, which process comprises

-   -   providing a paper coating composition according to the         invention; and     -   applying the paper coating composition to at least one surface         of paper or board.

The paper coating composition is preferably applied to uncoated base papers or uncoated board. The amount is generally in the range from 1 to 50 g and preferably in the range from 5 to 30 g (in terms of solids, i.e., without water or other solvents liquid at 21° C., 1 bar) per square meter. Coating can be effected by means of customary methods of application, for example via size press, film press, blade coater, air brush, doctor blade, curtain coating or spray coater. Depending on the pigment system, the paper coating compositions of the present invention can be used for the basecoat and/or for the topcoat.

Paper coating compositions according to the present invention have good performance characteristics. They have high binding power and high yellowing resistance. Papers coated with the paper coating compositions are readily printable in the customary printing processes, such as relief, gravure, offset, digital, inkjet, flexographic, newsprint, letterpress, sublimation, laser and electrophotographic printing or a combination thereof.

EXAMPLES

Unless the context suggests otherwise, percentages are always by weight. A reported content is based on the content in aqueous solution or dispersion.

Solids contents are determined by drying a defined amount of the particular aqueous polymeric dispersion (about 5 g) at 140° C. in a drying cabinet to constant weight. Two separate measurements are carried out in each case and averaged.

The average diameter of polymeric particles can be measured by hydrodynamic chromatography (HDC). In HDC, a colloidal sample elutes from a size-exclusion separation column sorted according to hydrodynamic radius. The eluent comprises salt, nonionic surfactants and anionic surfactants. Elution time is calibrated with PS calibration latices. The measurement range extends from 15 nm to 1200 nm—larger components are filtered out and not detected. Diameter and weight fractions can be measured to an accuracy of 3%. The fractions are weighted using the UV absorption at 254 nm.

Polymeric Dispersion D1

50% aqueous polymeric dispersion based on styrene/butadiene/acrylic acid/acrylamide/itaconic acid (52.8/41.8/3.5/1.3/0.6); particle size: 85 nm; Tg 0° C.

Polymeric Dispersion D2

50% aqueous polymeric dispersion based on styrene/butadiene/acrylic acid/acrylamide/itaconic acid (52.8/41.8/3.5/1.3/0.6); particle size: 95 nm; Tg 0° C.

Polymeric Dispersion D3

50% aqueous polymeric dispersion based on styrene/butadiene/acrylic acid/acrylamide/itaconic acid (52.8/41.8/3.5/1.3/0.6); particle size: 130 nm; Tg 0° C.

Polymeric Dispersion D4

50% aqueous polymeric dispersion based on styrene/butadiene/acrylic acid/itaconic acid (59/38/2/1); particle size: 215 nm; Tg 4° C.

Polymeric Dispersion D5:

Vinnacoat® 4433, 50% aqueous polymeric dispersion based on vinyl ester/ethylene particle size: 770 nm; Tg 11° C.

Paper Coating Composition:

The coating slip is prepared in a stirred assembly (Deliteur) into which the individual components were fed in succession. The pigments are added in pre-dispersed form (as a slurry). The other components are added after the pigments, the order corresponding to the order in the reported coating slip recipe. The final solids content is set by adding water.

Five paper coating slip compositions P1 to P5 were obtained using the 5 polymeric dispersions D1 to D5, composed as follows:

100 parts by weight of Hydrocarb® 60 slurry (finely divided calcium carbonate),

0.35 part by weight of Sterocoll® FS (acrylate-based thickener),

8 parts by weight of one of polymeric dispersions D1 to D5.

Solids content: 65% by weight.

A Magnostar® 58 g/m²a paper is one-sidedly coated with the coating slip in a semicommercial coating machine and calendared. The weight of the slip coat layer applied was 11 g/m².

The coated paper was tested for surface resistance using test methods known to a person skilled in the art. The following test methods were used:

IGT dry pick resistance

IGT wet pick resistance

Offset test

Yellowing resistance.

The results are summarized in Tables 1 and 2.

Measurement of Dry Pick Resistance Using IGT Tester (IGT Dry)

Strips were cut out of the in-test papers and printed using the IGT tester. The printing inks used are specialty test inks from Lorillieux, which transmit different tensile forces. The test strips are fed through the press at continuously increasing speed (maximum speed 200 cm/s). For evaluation, the point at which 10 picks have occurred on the paper surface after the start of printing is determined on the sample printing strip. The measure reported for dry pick resistance is the speed in cm/s present at this point during printing and also the test ink used. The higher this printing speed at the tenth pick point, the better the quality rating of the paper surface.

Measurement of Wet Pick Resistance with IGT Tester (IGT Wet)

Strips were cut out of the in-test papers and printed using the IGT tester. The tester was set up such that the test strips are moistened with water before printing. The printing inks used are specialty test inks from Lorilleux (No. 3807), which transmit different tensile forces. The print is performed at a constant speed of 0.6 cm/s. Picks from the paper surface are visible as unprinted areas. To determine wet pick resistance, an ink densitometer is used to determine ink density as a %age of the full hue. The higher the reported ink density, the better the wet pick resistance.

Offset Test:

Samples having a size of 240×46 mm are cut out of the in-test papers in the longitudinal direction. An appropriate amount of printing ink is applied to the inking roll and left to run for 1 minute. A printing disk is then inserted and inked for 30 s. The printing speed is 1 m/s. A paper strip is brought back to the starting position on a printing test support with the printed paper strip. After a specified time interval, the printing process is started again without replacing the printing disk. This operation is repeated more than once. After each printing cycle, the pick on the printed side of the paper strip is assessed by visual inspection. The table reports the number of cycles before picking occurred for the first time. The higher the number of cycles up to the occurrence of picking, the better the suitability of the papers for offset printing.

Test for Yellowing Resistance

UV Yellowing

The in-test papers, which initially all have similar CIE whiteness, are exposed to the light of a UV lamp. The whiteness of the paper is remeasured after certain periods. The CIE whiteness after 8 hours' exposure to light is reported.

Thermal Yellowing

The in-test papers, which initially all have similar CIE whiteness, are suspended in a drying cabinet at 120° C. The whiteness of the paper is remeasured after certain periods. The CIE whiteness after 48 hours' storage at 120° C. is reported.

TABLE 1 Binding power measurements Drying pick Wet pick Particle size resistance resistance Offset Example [nm] [cm/s] [%] passes P1 85 61 80 6.5 P2 95 55 74 6.25 P3 130 54 73 5.5 P4 215 49 65 4.5 P5 770 19 7 2.0

Coating slips P1 to P3, prepared using polymeric dispersions having an average particle size below 150 nm lead to higher binding power.

TABLE 2 Yellowing test results CIE whiteness CIE whiteness after 48 h Example after 8 h irradiation heating to 120° C. P1 67 45 P2 68 49 P3 68 45 P4 68 49 P5 72 60

Coating slips prepared using polymer dispersions based on ethylene-vinyl acetate copolymers (P5) lead to a higher resistance to yellowing. 

1. A paper coating composition, comprising (i) at least one inorganic pigment; and (ii) an aqueous polymeric dispersion comprising dispersed polymeric particles, wherein: the polymer of the polymeric particles is obtainable by polymerization of (a) at least one first monomer that is liquid at room temperature (20° C.), has a boiling point of at least 50° C. and a glass transition temperature of at least 20° C. as homopolymer, and (b) at least one second monomer that is gaseous at room temperature (20° C.), has a boiling point of below 0° C. and a glass transition temperature of below −30° C. as homopolymer; and, the dispersed polymeric particles have an average size of below 150 nm and a glass transition temperature in the range from −10 to +30° C.
 2. The paper coating composition according to claim 1, wherein the polymeric particles have an average size of 80 nm to below 150 nm.
 3. The paper coating composition according to claim 1, wherein: the at least one first monomer (a) is at least one of a vinylaromatic compound and a vinyl ester; and the at least one second monomer (b) is a mono- or poly-ethylenically unsaturated C2 to C4 hydrocarbon.
 4. The paper coating composition according to claim 1, wherein: the at least one first monomer (a) is at least one of styrene and vinyl acetate; and the at least one second monomer (b) is at least one of 1,3-butadiene and ethylene.
 5. The paper coating composition according to claim 1, wherein the polymer is selected from the group consisting of a polymer formed from ethylene and vinyl acetate to an extent of at least 60% by weight, a polymer formed from styrene and vinyl acetate to an extent of at least 60% by weight, and mixtures thereof.
 6. The paper coating composition according to claim 1, wherein the polymer is formed either from (a) 4.8 to 95 parts by weight of styrene or methylstyrene, (b) 4.8 to 95 parts by weight of butadiene, (c) 0.1 to 10 parts by weight of at least one ethylenically unsaturated acid, and (d) 0 to 15 parts by weight of at least one other monoethylenically unsaturated monomer, or from (a) 4.8 to 95 parts by weight of vinyl acetate, (b) 4.8 to 95 parts by weight of ethylene, and (c) 0 to 10 parts by weight of at least one other monoethylenically unsaturated monomer, such that the parts by weight of monomer each sum to
 100. 7. The paper coating composition according to claim 1, wherein the polymer is formed either from (a) 9.8 to 75 parts by weight of styrene or methylstyrene, (b) 24.8 to 90 parts by weight of butadiene, (c) 0.1 to 10 parts by weight of at least one ethylenically unsaturated acid, and (d) 0 to 15 parts by weight of at least one other monoethylenically unsaturated monomer, or from (a) 39.8 to 85 parts by weight of vinyl acetate, (b) 14.8 to 60 parts by weight of ethylene, and (c) 0 to 10 parts by weight of at least one other monoethylenically unsaturated monomer, such that the parts by weight of monomer each sum to
 100. 8. The paper coating composition according to claim 1, wherein an amount of the polymer in the aqueous polymeric dispersion is 1 to 50 parts by weight, based on the total amount of the at least one pigment which is present in an amount of 80 to 95 parts by weight, based on a total solids content.
 9. The paper coating composition according to claim 1, wherein the at least one pigment is selected from the group consisting of calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate, calcium carbonate, a silica, an alumina, aluminum hydrate, a silicate, titanium dioxide, zinc oxide, kaolin, argillaceous earth, talc and silicon dioxide.
 10. The paper coating composition according to claim 1, further comprising at least one assistant selected from the group consisting of a thickener, a further polymeric binder, a cobinder, an optical brightener, a filler, a flow control agent, a dispersant, a surfactant, a slip agent, a neutralizing agent, a defoamer, a deaerator, a preservative and a dye.
 11. A paper or board coated with the paper coating composition of claim
 1. 12. A process for coating paper or board, the process comprising: applying the paper coating composition of claim 1 to at least one surface of a paper or a board. 